Due to different preparation processes, the consistency of lithium-ion battery separators can vary significantly. Such consistency includes intrinsic properties (e.g., shutdown temperature) and apparent consistency (e.g., pore uniformity and thickness consistency observed under an electron microscope). # Physicochemical Properties of Lithium-Ion Battery Separators 1.  **Wettability and wetting rate**    Poor wettability of the separator will increase the internal resistance of the separator and the lithium-ion battery, which impairs the battery’s cycle performance and charge-discharge efficiency. The wetting rate of the separator refers to the speed at which the electrolyte penetrates into the micropores of the separator, which is related to the separator’s surface energy, pore size, porosity, tortuosity, and other characteristics. 2.  **Liquid absorption rate**    Since lithium-ion battery separator materials function as carriers for the electrolyte, they must meet the following requirements: ensuring unobstructed ion transport channels, and having an appropriate liquid absorption rate. In battery systems, various side reactions are inevitable, which consume a large amount of electrolyte. Therefore, the separator must have sufficient electrolyte retention capacity; otherwise, it will lead to insufficient electrolyte, increased interfacial resistance, and accelerated electrolyte consumption, forming a vicious cycle. Thus, the liquid absorption rate of the separator is a crucial parameter. 3.  **Chemical stability**    The separator should maintain long-term stability in the electrolyte and must not react with the electrolyte or electrode materials under strong redox conditions. The chemical stability of the separator is evaluated by measuring the electrolyte corrosion resistance and the swelling-shrinking rate of the separator in the electrolyte. 4.  **Thermal stability**    Lithium-ion batteries generate heat during charge-discharge processes; in particular, a large amount of heat will be released in case of short circuit or overcharge. Therefore, when the temperature rises, the separator should maintain its original integrity and a certain level of mechanical strength, continue to separate the positive and negative electrodes, and prevent short circuits. 5.  **Separator resistance**    The separator resistance is essentially the resistance of the electrolyte filled in the separator’s micropores. It is related to many factors, including porosity, pore tortuosity, electrolyte conductivity, separator thickness, and the wettability of the separator material by the electrolyte. The alternating current (AC) impedance method is commonly used to test separator resistance. However, due to the thinness of the separator and the frequent presence of defects, the measurement results are prone to errors. Therefore, multi-layer samples are often used for testing, and the average value is taken as the final result. 6.  **Self-shutdown performance**    When the temperature exceeds a certain threshold, exothermic reactions occur inside the lithium-ion battery, leading to self-heating. In addition, charger malfunctions, safety current failures, and other issues can cause battery overcharge or external short circuit, all of which will generate a large amount of heat. As separators are typically made of thermoplastic polyolefin materials, when the temperature approaches the melting point of the polymer, the porous ion-conductive membrane of the polymer will close its pores and turn into a non-porous insulating layer, resulting in a shutdown effect. This blocks the continuous conduction of ions to form a circuit, thereby protecting the battery. Therefore, polyolefin separators can provide additional safety protection for lithium-ion batteries. The primary function of the separator is to separate the positive and negative electrodes of a lithium-ion battery to prevent direct contact and short circuit between them. In addition, it also serves as a channel for electrolyte ions to pass through. It is well known that the quality of the separator directly affects the capacity, cycle performance, safety performance, and other characteristics of lithium-ion batteries. The excellent performance of the separator plays an important role in improving the overall performance of the battery.